1. Field of the Invention
This invention relates to an off-the-road heavy duty pneumatic radial tire for use in large vehicles running on irregular ground such as a building site, rough ground surface for transportation and work in mines or the like. More particularly it pertains to an off-the-road heavy duty pneumatic radial tire having an effective and appropriate means for mitigating the occurrence of inner rubber peeling and cracking in the vicinity of the shoulder portion of the tread.
2Related Art Statement
In general, the improvement of wear resistance, low heat build-up and low rolling resistance required in radial tires is attempted by arranging a belt having a high rigidity between the carcass and the tread in a radial tire.
However, since the rigidity of the belt is high, large size radial tires mainly running on rough road are high in the load bearing ratio at the same internal pressure even at standard or actual use states as compared with tires running on good road such as paved road or the like at a high speed. Such tires have many chances of being subjected to impact loading through unevenness on rough roads, so that excessive strain-stress are always applied to the belt in the vicinity of the free end of the belt. Consequently the peeling is caused at the end the belt to finally facilitate the occurrence of cracking.
That is, the carcass line of the radial tire is fundamentally an equilibrium shape so that a distance from a base line passing through a bead heel which is located from a rotational axis of the tire by 1/2 of nominal diameter of a rim mounting the tire and in parallel to the rotational axis of the tire is maximum at an equatorial plane of the tire and reduces toward a position corresponding to the maximum width of the tire. Furthermore, the belt is fundamentally arranged along the carcass line or substantially in parallel to the base line.
In such carcass and belt shapes, the tension bearing ratio of the belt is increased near to the belt end at a time of filling an internal pressure and at a time of deformation under a load, so that the influence of strain-stress of rubber in the vicinity of the belt end considerably influences durability of the tire.
In this connection, U.S. Pat. No. 4,481,994 is concerned with a so-called controlled carcass line shifted from a natural equilibrium shape of the carcass line and discloses that the carcass line before filling of the internal pressure is arranged outside both the radial and axial directions of the tire between a position corresponding to the maximum width of the tire and a position in parallel to the belt as compared with the carcass line at the filling of internal pressure. Also the carcass line before filling of internal pressure is arranged outside the radial direction of the tire and inside the axial direction thereof between the position corresponding to the maximum width of the tire and bead core as compared with the carcass line at the filling of internal pressure. In the tire using such a controlled carcass line, however, the strain-stress at the belt end is slightly improved at the filling of internal pressure, but it is still insufficient for improvement of the belt durability in a off-the-road heavy duty pneumatic radial tire which is the subject of this invention.
Because, the higher durability of the tire is required as the service conditions of the off-the-road tire, particularly input condition from uneven ground surface to tread with the advance of high speed running requirement become more severer, and consequently it becomes important to avoid the occurrence of peeling failure in the vicinity of the belt end. In order to prevent the occurrence of the peeling failure in the vicinity of the belt end, it is naturally required to sufficiently reduce minute motion generated at the belt end, i.e. stress produced by internal strain. Such an internal strain-stress is produced when the tire tread grows in the radial direction thereof at the filling of internal pressure.
On the other hand, the deformation strain-stress is produced by a loading applied to the tire tread during the running of the tire under a load and a force input to the tread through unevenness on irregular ground.
At the present time, it can be said that the total control of initial strain-stress and deformation strain-stress generated at the belt end under the severer service conditions as mentioned above is not still developed.
As to the reduction of the initial strain-stress and deformation strain-stress in the vicinity of the belt end, the inventors have previously attempted to render the belt of the tire into a concave form outward in the radial direction of the tire before the filling of internal pressure (hereinafter referred to as belt concave simply) to thereby obtain proper change quantity of each part of the belt after the filling of internal pressure. This is as reported in Japanese Patent laid open No. 61-157403, and consequently it has been found that the effect of improving the durability of the belt is developed as compared with the conventional tire but is insufficient under recent severer service conditions because there is a restriction on the effect.